BACKGROUND: Chikungunya virus (CHIKV) is transmitted through the bite of infected Aedes aegypti or Aedes albopictus mosquitoes and humans serve as amplifying reservoirs during outbreaks. CHIKV causes chikungunya fever (CHIK), which is characterized by fever and joint pain. CHIK had been rarely reported in Pennsylvania prior to 2014. Ae. albopictus occurs in the state, but local transmission of CHIKV has never been documented. Unprecedented CHIKV transmission in the Americas throughout 2014 led to an increase in Pennsylvanian travelers returning home with CHIK.
METHODS: We summarized epidemiologic and reporting characteristics of probable and confirmed CHIK cases reported to the Pennsylvania Department of Health (PADOH) in 2014. We also analyzed the risk of local transmission in Pennsylvania. CHIK cases were defined as viremic in Pennsylvania (VPA-CHIK) if they returned to Pennsylvania < 7 days of onset. Mosquito collection data from 2014 were obtained from the Pennsylvania Department of Environmental Protection (PADEP). We mapped Ae. albopictus collection sites, and compared them with VPA-CHIK case residence location.
RESULTS: During 2014, PADOH received 97 imported CHIK cases. Ninety-five (97.9%) reported recent travel to the Caribbean, Central America, or South America; two had traveled to Asia. Of cases with known travel and onset dates, 58 (74.4%) were considered VPA-CHIK. Illness onset occurred from June–September for 48 (82.8%) VPA-CHIK cases. Also from June–September, PADEP collected 17,791 Ae. albopictus specimens from 1,114 sites in 23 counties. Fifty-three (91.4%) VPA-CHIK cases resided in a county where Ae. albopictus was collected. The median distance between a VPA-CHIK case and an Ae. albopictus collection site was 0.48 miles. The median time from VPA-CHIK case symptom onset to PADOH report receipt was 22.1 days and the median time from VPA-CHIK case symptom onset to specimen collection was 4.0 days.
CONCLUSIONS: Pennsylvania experienced a surge of imported CHIK in 2014 as a result of extensive CHIKV transmission in the Americas. Our review identified several potential risk factors for infection of local mosquitoes with CHIKV, including: numerous travelers returning home while potentially viremic, a large proportion of cases with onset and residence spatiotemporally consistent with Ae. albopictus, and delays in reporting cases to PADOH. Although the CHIKV genotype responsible for the epidemic in the Americas is more efficiently transmitted by Ae. aegypti, providers should be educated on recognition and diagnosis of CHIK and encouraged to report suspected cases to public health officials when travel history and symptoms are consistent with CHIK.